This module provides support for raising and catching both built-in
and user-defined exceptions.

In addition to exceptions thrown by IO operations, exceptions may
be thrown by pure code (imprecise exceptions) or by external events
(asynchronous exceptions), but may only be caught in the IO monad.
For more details, see:

The type of exceptions. Every kind of system-generated exception
has a constructor in the Exception type, and values of other
types may be injected into Exception by coercing them to
Dynamic (see the section on Dynamic Exceptions:
Control.OldException).

The ExitException exception is thrown by System.Exit.exitWith (and
System.Exit.exitFailure). The ExitCode argument is the value passed
to System.Exit.exitWith. An unhandled ExitException exception in the
main thread will cause the program to be terminated with the given
exit code.

An attempt was made to invoke a class method which has
no definition in this instance, and there was no default
definition given in the class declaration. GHC issues a
warning when you compile an instance which has missing
methods.

NonTermination

The current thread is stuck in an infinite loop. This
exception may or may not be thrown when the program is
non-terminating.

A field selection was attempted on a constructor that
doesn't have the requested field. This can happen with
multi-constructor records when one or more fields are
missing from some of the constructors. The
String argument gives the location of the
record selection in the source program.

An attempt was made to update a field in a record,
where the record doesn't have the requested field. This can
only occur with multi-constructor records, when one or more
fields are missing from some of the constructors. The
String argument gives the location of the
record update in the source program.

The current thread's stack exceeded its limit.
Since an exception has been raised, the thread's stack
will certainly be below its limit again, but the
programmer should take remedial action
immediately.

HeapOverflow

The program's heap is reaching its limit, and
the program should take action to reduce the amount of
live data it has. Notes:

Although throwIO has a type that is an instance of the type of throw, the
two functions are subtly different:

throw e `seq` x ===> throw e
throwIO e `seq` x ===> x

The first example will cause the exception e to be raised,
whereas the second one won't. In fact, throwIO will only cause
an exception to be raised when it is used within the IO monad.
The throwIO variant should be used in preference to throw to
raise an exception within the IO monad because it guarantees
ordering with respect to other IO operations, whereas throw
does not.

throwTo does not return until the exception has been raised in the
target thread.
The calling thread can thus be certain that the target
thread has received the exception. This is a useful property to know
when dealing with race conditions: eg. if there are two threads that
can kill each other, it is guaranteed that only one of the threads
will get to kill the other.

If the target thread is currently making a foreign call, then the
exception will not be raised (and hence throwTo will not return)
until the call has completed. This is the case regardless of whether
the call is inside a block or not.

Important note: the behaviour of throwTo differs from that described in
the paper "Asynchronous exceptions in Haskell"
(http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm).
In the paper, throwTo is non-blocking; but the library implementation adopts
a more synchronous design in which throwTo does not return until the exception
is received by the target thread. The trade-off is discussed in Section 9 of the paper.
Like any blocking operation, throwTo is therefore interruptible (see Section 5.3 of
the paper).

There is currently no guarantee that the exception delivered by throwTo will be
delivered at the first possible opportunity. In particular, a thread may
unblock and then re-block exceptions (using unblock and block) without receiving
a pending throwTo. This is arguably undesirable behaviour.

This is the simplest of the exception-catching functions. It
takes a single argument, runs it, and if an exception is raised
the "handler" is executed, with the value of the exception passed as an
argument. Otherwise, the result is returned as normal. For example:

For catching exceptions in pure (non-IO) expressions, see the
function evaluate.

Note that due to Haskell's unspecified evaluation order, an
expression may return one of several possible exceptions: consider
the expression error "urk" + 1 `div` 0. Does
catch execute the handler passing
ErrorCall "urk", or ArithError DivideByZero?

The answer is "either": catch makes a
non-deterministic choice about which exception to catch. If you
call it again, you might get a different exception back. This is
ok, because catch is an IO computation.

Note that catch catches all types of exceptions, and is generally
used for "cleaning up" before passing on the exception using
throwIO. It is not good practice to discard the exception and
continue, without first checking the type of the exception (it
might be a ThreadKilled, for example). In this case it is usually better
to use catchJust and select the kinds of exceptions to catch.

Also note that the Prelude also exports a function called
Prelude.catch with a similar type to catch,
except that the Prelude version only catches the IO and user
families of exceptions (as required by Haskell 98).

The function catchJust is like catch, but it takes an extra
argument which is an exception predicate, a function which
selects which type of exceptions we're interested in. There are
some predefined exception predicates for useful subsets of
exceptions: ioErrors, arithExceptions, and so on. For example,
to catch just calls to the error function, we could use

result <- catchJust errorCalls thing_to_try handler

Any other exceptions which are not matched by the predicate
are re-raised, and may be caught by an enclosing
catch or catchJust.

Similar to catch, but returns an Either result which is
(Right a) if no exception was raised, or (Left e) if an
exception was raised and its value is e.

try a = catch (Right `liftM` a) (return . Left)

Note: as with catch, it is only polite to use this variant if you intend
to re-throw the exception after performing whatever cleanup is needed.
Otherwise, tryJust is generally considered to be better.

Also note that System.IO.Error also exports a function called
System.IO.Error.try with a similar type to try,
except that it catches only the IO and user families of exceptions
(as required by the Haskell 98 IO module).

Forces its argument to be evaluated to weak head normal form when
the resultant IO action is executed. It can be used to order
evaluation with respect to other IO operations; its semantics are
given by

Because the Exception datatype is not extensible, there is an
interface for throwing and catching exceptions of type Dynamic
(see Data.Dynamic) which allows exception values of any type in
the Typeable class to be thrown and caught.

Asynchronous exceptions are so-called because they arise due to
external influences, and can be raised at any point during execution.
StackOverflow and HeapOverflow are two examples of
system-generated asynchronous exceptions.

throwTo (also throwDynTo and Control.Concurrent.killThread) allows one
running thread to raise an arbitrary exception in another thread. The
exception is therefore asynchronous with respect to the target thread,
which could be doing anything at the time it receives the exception.
Great care should be taken with asynchronous exceptions; it is all too
easy to introduce race conditions by the over zealous use of
throwTo.

Applying block to a computation will
execute that computation with asynchronous exceptions
blocked. That is, any thread which
attempts to raise an exception in the current thread with Control.Exception.throwTo will be
blocked until asynchronous exceptions are enabled again. There's
no need to worry about re-enabling asynchronous exceptions; that is
done automatically on exiting the scope of
block.

Threads created by Control.Concurrent.forkIO inherit the blocked
state from the parent; that is, to start a thread in blocked mode,
use block $ forkIO .... This is particularly useful if you need to
establish an exception handler in the forked thread before any
asynchronous exceptions are received.

There's an implied block around every exception handler in a call
to one of the catch family of functions. This is because that is
what you want most of the time - it eliminates a common race condition
in starting an exception handler, because there may be no exception
handler on the stack to handle another exception if one arrives
immediately. If asynchronous exceptions are blocked on entering the
handler, though, we have time to install a new exception handler
before being interrupted. If this weren't the default, one would have
to write something like

block (
catch (unblock (...))
(\e -> handler)
)

If you need to unblock asynchronous exceptions again in the exception
handler, just use unblock as normal.

Note that try and friends do not have a similar default, because
there is no exception handler in this case. If you want to use try
in an asynchronous-exception-safe way, you will need to use
block.

Some operations are interruptible, which means that they can receive
asynchronous exceptions even in the scope of a block. Any function
which may itself block is defined as interruptible; this includes
Control.Concurrent.MVar.takeMVar
(but not Control.Concurrent.MVar.tryTakeMVar),
and most operations which perform
some I/O with the outside world. The reason for having
interruptible operations is so that we can write things like

block (
a <- takeMVar m
catch (unblock (...))
(\e -> ...)
)

if the Control.Concurrent.MVar.takeMVar was not interruptible,
then this particular
combination could lead to deadlock, because the thread itself would be
blocked in a state where it can't receive any asynchronous exceptions.
With Control.Concurrent.MVar.takeMVar interruptible, however, we can be
safe in the knowledge that the thread can receive exceptions right up
until the point when the Control.Concurrent.MVar.takeMVar succeeds.
Similar arguments apply for other interruptible operations like
System.IO.openFile.

If the first argument evaluates to True, then the result is the
second argument. Otherwise an AssertionFailed exception is raised,
containing a String with the source file and line number of the
call to assert.

Assertions can normally be turned on or off with a compiler flag
(for GHC, assertions are normally on unless optimisation is turned on
with -O or the -fignore-asserts
option is given). When assertions are turned off, the first
argument to assert is ignored, and the second argument is
returned as the result.

When you want to acquire a resource, do some work with it, and
then release the resource, it is a good idea to use bracket,
because bracket will install the necessary exception handler to
release the resource in the event that an exception is raised
during the computation. If an exception is raised, then bracket will
re-raise the exception (after performing the release).